This invention relates generally to control systems and more particularly to a system and method for controlling an oscillator.
Oscillators, such as voltage-controlled crystal oscillators (VCXOs), are widely used in many telecommunications systems. One common use for such oscillators is in clock recovery applications. In particular, oscillators may be used to clean an incoming clock signal in order to better ensure accuracy in reading data transmitted in telecommunications systems.
As the components utilized in telecommunications systems become smaller and more integrated customers want to package as many devices as possible into a single chip solution. For example, network switching components, laptop computers, and cellular telephones are becoming more and more integrated with each successive generation of hardware releases.
Many oscillators currently used in clock recovery applications include an external (off-chip) varactor to vary the phase shift introduced in a voltage-controlled crystal oscillator. Such a phase shift is utilized in order to properly adjust a new frequency for a clock signal during cleaning. The external varactors currently in use may be expensive, may not be easily integrated into standard integration processes or application-specific integrated circuits (ASICS), and may require the use of higher voltage power sources.
In accordance with the present invention, a system and method for controlling an oscillator are provided that substantially reduce disadvantages and problems associated with previously developed systems and methods.
In one embodiment of the present invention, an oscillator is disclosed for controlling the frequency of an output clock signal in response to detecting an error in the frequency of an input clock signal. The oscillator includes an inverter operable to generate a voltage signal and a resonator coupled to the inverter operable to introduce a phase shift in the voltage signal. The oscillator also includes a variable resistor positioned across a feedback path of the inverter and operable to introduce a further phase shift in the voltage signal in response to the detected error. The resonator is further operable to adjust the frequency of the voltage signal in response to the introduced further phase shift. The voltage signal is used as the output clock signal.
In a second embodiment, a method of adjusting the frequency of a clock signal generated by an oscillator is disclosed. The method includes receiving a voltage input indicative of an error in the frequency of the clock signal and adjusting a feedback impedance across an inverter in response to the received voltage input. The method further includes introducing a phase shift in the voltage output in response to the adjusted feedback impedance and adjusting the frequency of the clock signal in response to the introduced phase shift.
In a third embodiment, a phase-locked loop for adjusting the frequency of a clock signal is disclosed. The phase-locked loop includes a detector operable to detect an error in the frequency of the clock signal and an oscillator coupled to the detector and operable to adjust the frequency of the clock signal in response to the detected error. The oscillator also includes a resonator coupled to an inverter and operable to adjust the frequency of the clock signal in response to a phase shift introduced in a voltage signal by a variable resistor positioned across a feedback path of the inverter.
Technical advantages of the present invention include providing an improved system and method for controlling an oscillator. In particular, embodiments of the present invention may eliminate the additional expense of an external varactor. Additionally, various embodiments of the present invention may allow a system for controlling an oscillator to be more easily integrated with many telecommunications and wireless system applications. A further advantage of various embodiments of the present invention is to allow the use of a lower voltage power supply to control oscillation. Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.